Wide band transistor video signal amplifier



1957 B. s. VILKOMERSON 3,3

WIDE BAND TRANSISTOR VIDEO SIGNAL AMPLIFIER Filed May 15, 1965 Pryor /4/f Nat [Z 25 NVENTOR. #1915079)? United States Patent 3,356,959 WIDE BAND TRANSISTOR VIDEO SIGNAL AMPLIFIER Benjamin S. Vilkomerson, Camden, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed May 13, 1965, Ser. No. 455,502

6 Claims. (Cl. 330-28) ABSTRACT OF THE DISCLOSURE This invention relates to transistorized signal amplifiers and particularly to relatively wide band video signal ampli' fiers.

In television studies it is necessary to convey relatively wide band video and other relatively high frequency signals between numerous points such as camera locations, video signal control consoles, video signal distribution amplifiers and the like. Transmission paths such as coaxial cables usually are used for such purposes. These cables are capable of conveying the video signals from one point to another without serious frequency selective distortion due to standing waves. When long lengths of such cable are used, however, the video signals are attenuated differently at low and high frequencies. It, therefore, is the practice to employ video signal amplifiers for driving such coaxial lines in a manner to equalize, the anticipated signal attenuation so that the signals arriving at different points will have substantially their original relative amplitudes at both low and high frequencies.

In recent years in the transistorization of such line driving video amplifiers it has been common to provide as an output stage what has become known as a feedback pair of transistors. In such an arrangement a negative signal feedback path is provided from the output transistor to the input transistor so that greater bandwidth is achieved and transistors may be interchanged and/ or replaced without significantly altering the overall gain of the output stage. Such an arrangement is economically feasible because relatively inexpensive transistors may be employed and even though their amplifying properties may vary appreciably, the negative feedback arrangement enables the stage to operate with a substantially uniform and predictable gain.

The output of such a feedback pair is capacitively coupled to the input of a coaxial cable which is terminated at both ends in its characteristic impedance. It is customary to employ a relatively low impedance cable such as a suitable length of 50 ohm cable for example. In order that the video signal derived from the video amplifier have substantially the same flat response characteristic down to the lowest signal frequencies as applied to the transmission line, it has been necessary to employ a coupling capacitor having a capacitance of perhaps 10,000 to 20,000 microfarads, for example. There are many disadvantages in the use of such a coupling capacitor such as its large size, its weight and its distributed capacitance to ground. Other arrangements have been proposed for overcoming some of these disadvantages whereby to enable the use of a smaller coupling capacitor. Although one such alternative arrangement overcomes someof the disadvantages and has been successfully used in certain apparatus, it does not provide optimum results in all cases.

It, therefore, is an object of the present invention to provide a video signal amplifier with a coupling arrangement to an output circuit which does not require the use of an objectionably large coupling capacitor.

In accordance with the present invention the video amplifier is provided with a feedback pair of transistors in which there is embodied a DC or low frequency feedback path including a resistor from the output transistor and an AC or high frequency feedback path including a capacitor taken from the output terminal of the capacitor coupling the amplifier to an output circuit. The high and low frequency feedback paths are coupled through a feed back transistor to the input transistor of the feedback pair. The signal feedback circuit is degenerative so that the advantages of the use of a feedback pair of transistors is obtained. Sufiicient amplitude of the negative feedback principally of the DC or low frequency component of the video signal to the input transistor is obtained by utilizing the amplification capability of the feedback transistor. In this way the coupling capacitor between the output transistor and the utilization circuit and the feedback capacitor in the AC or high frequency feedback path may be of relatively small sizes. At the same time the advantages of a feedback pair of transistors is retained so as to provide a video signal to the utilization circuit which has the desired amplitude and which has a relatively uni form response at all signal frequencies.

For a more complete description of the invention and its relationship to the prior art reference may be made to the following description which is taken in conjunction with the accompanying drawings of which:

FIGURE 1 is a schematic circuit diagram of a prior art video amplifier for driving a transmission line;

FIGURE 2 is a schematic circuit diagram of a video signal line driver amplifier which has been proposed as an improvement over the arrangement of FIGURE 1; and

FIGURE 3 is a schematic circuit diagram embodying the present invention comprising a still further improvement over the arrangement of FIGURE 1.

In FIGURE 1 video signals are applied to terminal 11. These signals may be relatively wide band video signals having frequencies ranging from substantially direct current to approximately 10 megacycles and may also include other information such as blanking and synchronizing pulses. The signals appearing at terminal 11 are impressed by a series connected resistor 12 upon the base electrode of an input transistor 13, the emitter electrode of which is grounded and the collector electrode of which is connected through a load resistor 14 to a terminal 15 to which is applied a suitable source of voltage +E. An output transistor 16 has its base electrode connected to the collector electrode of transistor 13 and its collector electrode connected to the voltage terminal 15, the emitter electrode being connected through a feedback resistor 17 to the base electrode of the input transistor 13. A resistor 18 connected between the base electrode of the input transistor 13 and a E voltage terminal 19 provides suitable biasing of the input transistor. A resistor 21 connected between the emitter electrode of the output transistor 16 and the E voltage terminal 19 comprises the load for the output transistor 16.

By means of the described circuits interconnecting the input and output transistors 13 and 16 respectively the transistors comprise a feedback pair, the signal feedback from the output of the transistor 16 to the input of the transistor 13 being negative or degenerative. Such an arrangement is commonly used so that such an amplifying stage will have a substantially flat response over a wide frequency range and a substantially constant gain irrespective of even large variations in the amplification factors or Betas of the transistors. This arrangement enables the substitution or replacement of transistors without materially changing the gain or bandwidth of the stage.

The output video signal which is developedat the emitter electrode of the output transistor 16 is coupled by a capacitor 22 and a series connected resistor 23 to the input of a relatively low impedance transmission line such as the coaxial cable 24, the other end of which is terminated by a resistor 25 having a value equal to the characteristic impedance of the cable. The cable, particularly as used in television studios, may have different lengths which in some instances may be hundreds of feet. One of the main purposes of such a line driving video amplifier is to impress a substantially undistorted wide band video signal upon the cable 24 in such amplitude that, after undergoing attenuation in the cable, there is produced at a terminal 26 a substantially undistorted wide band video signal of a predetermined amplitude for connection from the terminal 26 to suitable utilization apparatus such as video signal distribution amplifiers and the like.

In order to effect uniform gain by an amplifier arrangement such as that shown in FIGURE 1 the video signal which is developed at the emitter electrode of the output transistor 16 is degeneratively coupled back to the base electrode of the input 13 by means of the resistor 17. Both low frequency and high frequency components of the video signal thus are fed back without any substantial mutual discrimination. The gain of the amplifier is determined by the ratio of resistor 17 to resistor 12. Such an arrangement, however, when used to drive the connecting cable 24 requires the use of a coupling capacitor 22 which has a value of the order of 20,000 microfarads in order that there be substantially no discrimination of even the lowest frequency components of the signal impressed upon the cable 24. The use of such a coupling capacitor, however, has several disadvantages such as the relatively large size and weight of the capacitor and the attendant distributed capacitance to ground.

FIGURE 2 illustrates a circuit, representing an improvement over that of FIGURE 1, of a feedback pair of transistors which enables the use of an output coupling capacitor 22a of a considerably smaller size than the corresponding capacitor 22 of FIGURE 1. Circuit components of FIGURE 2 which are the same as or similar to corresponding components in FIGURE 1 are designated by the same reference characters. In FIGURE 2 a high frequency signal feedback capacitor 27 is connected from the output side of the coupling capacitor 22a to the junction point between the feedback resistor 17 and an isolating resistor 28 connected to the emitter electrode of the output transistor 16. By such an arrangement there is provided an AC feedback path from the output transistor 16 to the input transistor 13 which includes the capacitor 27. By reason of the use of the smaller size capacitor 22a low frequency components of the video signal appearing at the output side of the coupling capacitor 22a are considerably attenuated by reason of the higher impedance presented to such frequencies by the capacitor 22a. As a result, there is less low frequency current fed back via the capacitor 27 to the base electrode of the input transistor 13 so that the overall gain of the amplifier at such low frequencies is increased, thereby to a great extent offsetting the described reduction in the output of the low frequency signal components. In such an arrangement the gain of the amplifier for high and medium frequency components of the video signal is determined as in the previously described arrangement of FIGURE 1 by the ratio of the value of resistor 17a to the value of resistor 12. The gain of the amplifier for the lowest video frequencies is determined by the ratio of the sum of resistors 17a and 28 to the resistor 12. In order to obtain good high frequency response by such an amplifier (e.g.

an adequately high voltage output from the amplifier it is necessary that the currents representing both low frequency and high frequency video signal components present in the feedback resistor 17a must be of fairly large amplitudes. Hence, in order to obtain good low frequency as wellas high frequency operation, the value of the feedback capacitor 27 must be large enough so that its reactance at the low frequencies is negligible compared to the values of resistors 17a and 18. This is necessary in order to keep the feedback currents in the two paths of appropriate magnitudes so that the feedback current to the input transistor 13 is proportional to and in phase with the output voltage developed at the emitter electrode of the transistor 16. If, in order to achieve the desired results, the values of the feedback capacitor 27 and the isolating resistor 28 are made too small, the response by the amplifier to the low frequency video component is not faithful because the feedback current to the input transistor 13 will be supplied not only through the capacitor 27 but also through the resistor 28. In order to avoid such an occurrence it is necessary that the capacitor 27 have a value of several thousand microfarads. If, on the other hand, the resistor 28 is made large enough to reduce the undesired infidelity of low frequency response and a moderate sized feedback capacitor 27 is used, then the gain of the amplifier at very low frequencies becomes too great. This may be seen by considering, as previously noted, that the gain at the lowest frequencies is determined by the ratio of both resistors 17a and 28 to the value of resistor 12. Such a condition, particularly when combined with phase shifts which are likely to be encountered, often results in very low frequency oscillation of the amplifier of the type known as motor boating.

FIGURE 3 illustrates an embodiment of the present invention which constitutes an improvement over the previously described arrangements of FIGURES 1 and 2. In this figure those components which correspond to similar components of FIGURES 1 and 2 are designated by the same reference characters. An additional transistor 29 is provided in the feedback path from the output tran: sistor 16 to the input transistor 13. A capacitor 27a and a resistor 28a are coupled to the base of the feedback transistor 29 and constitute respectively AC and DC feedback circuits. These two components are coupled to receive signals from the same circuit points as the corresponding elements in FIGURE 2. The feedback transistor 29 is connected as an emitter follower having a load resistor 31 coupled between the emitter electrode and the -E voltage terminal 19. The emitter electrode of this transistor also is connected through a combined AC and DC feedback resistor 17b to the base of the input tran sistor 13.

In this embodiment of the invention the feedback capacitor 27a may be made considerably smaller than the feedback capacitor 27 of FIGURE 2 and the resistor 28a may be made considerably larger than the corresponding resistor 28 of FIGURE 2. In effect, the capacitance of the capacitor 27a is multiplied by the Beta of the feedback transistor 29. The alternating current through the feedback resistor 17b which is derived from the emitter electrode of the transistor 29 is Beta times as large as the AC component of the signal applied to the base of the transistor 29 through the feedback capacitor 27a. Consequently, for a desired value of feedback current at very low frequencies the value of the capacitor 27a need be only that represented by 1/ ,8 as large as would be required in the absence of the transistor 29. For example, if the transistor 29 has a Beta of 100, the capacitor 27a need only be a 300 microfarad in order to provide a performance equivalent to one having a 30,000 microfarad capacitance in the circuit of FIGURE 2, for example.

The current which constitutes a DC stabilizing bias applied to the base of the input transistor 13 through the feedback resistor 17b is produced by a current which is applied to the base of the feedback transistor 29 through the resistor 28a and has a value of only l/p times the value of the stabilizing current through the resistor 17b. Hence, for a given stabilizing bias current through the feedback resistor 17b, the resistor 28a may have a Value which is Beta times the value of resistor 28 in FIGURE 2, for example. As a result of the relatively large value of resistor 28a and the large effective value of capacitor 27a, any phase shift in the feedback loop is kept quite small, even at very low signal frequencies. Perhaps more importantly, the voltage gain of the feedback pair of transistors 13 and16 is not substantially increased even at very'low frequencies by reason of the use of the relatively high value of resistor 28a because the voltage appearing at the emitter electrode of the output transistor 16 at these very low frequencies is essentially duplicated at the emitter electrode of the feedback transistor 29. The only small difference in these two voltages is that represented by the inherent voltage drop between the base and emitter electrodes of the transistor 29. As a result, the voltage gain at these low signal frequencies is practically represented by the ratio of resistor 17b to resistor 12. It will be remembered that, in the case of the circuit shown in FIGURE 2', the voltage gain was determined by the ratio of the sum of resistors 17a and 28 to the resistor 12. By virtually eliminating any increase in the voltage gain of the amplifier at very low frequencies any tendency for the circuit to develop motor boating instabilities is eliminated.

Thus, the circuit of FIGURE 3 embodying the invention retains all of the beneficial features of a feedback pair of transistors such as a uniform response over a relatively wide band of frequencies and in addition enables the use of an output coupling capacitor of moderate value. The capacitors 22b and 27a may have values of only a few hundred microfarads, for example.

What is claimed is:

1. A video signal amplifier having substantially equal response for all video signal frequencies for driving a transmission line having an input terminal for conveying said video signal to utilization apparatus, comprising in combination:

a pair of transistors having input and output electrodes and a common input-output electrode;

said transistors being interconnected so that signals derived from said output electrode are of opposite phase to signals applied to said input electrode;

means for impressing upon the input electrode of said transistor pair a video signal having a relatively wide range of frequency components;

an output coupling capacitor having an input terminal connected to said transistor pair output electrode and an output terminal coupled to said transmission line;

a negative feedback circuit between the output and input electrodes of said transistor pair;

said circuit including a feedback transistor having input and output electrodes;

-DC and AC means respectively coupling the output electrode of said transistor pair and the output terminal of said coupling capacitor to said feedback transistor input electrode; and

means coupling said feedback transistor output electrode to said transistor pair input electrode.

2. A video signal amplifier having substantially equal response for all video signal frequencies for driving a transmission line having an input terminal for conveying said video signal to utilization apparatus, comprising in combination:

a pair of transistors having input and output electrodes and a common input-output electrode;

said transistors being interconnected so that signals derived from said output electrode are of opposite phase to signals applied to said input electrode;

means for impressing upon the input electrode of said transistor pair a video signal having a relatively wide range of frequency components;

an output coupling capacitor having an input terminal connected to said transistor pair output electrode and an output terminal coupled to the input terminal of said transmission line;

a degenerative feedback circuit between the output and input electrodes of said transistor pair;

said circuit including a feedback transistor having input and output electrodes;

means providing a DC coupling between the output electrode of said transistor pair and said feedback transistor input electrode;

means providing an AC coupling from the output terminal of said coupling capacitor to said feedback transistor input electrode; and

means coupling said feedback transistor output electrode to said transistor pair input electrode.

3. A video signal amplifier having substantially equal response for all video signal frequencies for driving a transmission line having an input terminal for conveying said video signal to utilization apparatus, comprising in combination:

input and output transistors each having input and output electrodes and a common input-output electrode;

means for impressing upon said input transistor input electrode a video signal having a relatively wide range of frequency components;

means coupling said input transistor output electrode to to said output transistor input electrode;

an output coupling capacitor having an input terminal connected to said output transistor output electrode and an output terminal coupled to the input terminal of said transmission line;

a feedback transistor having input and output electrodes;

a DC degenerative feedback path between said output transistor output electrode and said feedback transistor input electrode;

an AC degenerative feedback path from the output terminal of said coupling capacitor to said feedback transistor input electrode; and

a composite degenerative feed-back path between said feedback transistor output electrode and said input transistor input electrode.

4. A video signal amplifier having substantially equal response for a video signal including a relatively wide range of frequencies for driving a relatively low impedance transmission line having an input terminal for conveying said video signal to utilization apparatus, comprising in combination:

input and output transistors each having base, emitter and collector electrodes;

means for impressing upon said input transistor base electrode a video signal having a relatively wide range of frequency components;

means coupling said input transistor collector electrode to said output transistor base electrode;

an output coupling capacitor having an input terminal connected to said output transistor emitter electrode and an output terminal coupled to the input terminal of said transmission line;

a feedback transistor having base, emitter and collector electrodes;

a DC feed-back path including a resistor connected between said output transistor emitter electrode and said feedback transistor base electrode;

an AC feedback path including a capacitor connected from the output terminal of said coupling capacitor to said feedback transistor base electrode; and

a composite feedback path between said feedback transistor emitter electrode and said input transistor base electrode.

5. A video signal amplifier having substantially equal response for low, medium and high frequencies for driving a relatively low impedance transmission line having an input terminal for conveying said video signal to utilization apparatus, comprising in combination:

input and output transistors each having base, emitter and collector electrodes,

said input transistor being connected in a common emitter configuration and said output transistor being connected as an emitter follower;

means for impressing upon said input transistor base electrode a video signal having low, medium and high frequency components;

means coupling said input transistor collector electrode to said output transistor base electrode;

an output coupling capacitor having an input terminal connected to said output transistor emitter electrode and an output terminal coupled to the input terminal of said transmission line;

a feedback transistor having base, emitter and collector electrodes and connected as an emitter follower;

a DC feed-back path between said output transistor emitter electrode and said feedback transistor base electrode;

an AC feedback path from the output terminal of said coupling capacitor to said feedback transistor base electrode; and

a composite feedback resistor connected between said feedback transistor emitter electrode and said input transistor base electrode.

6. A video signal amplifier having substantially equal response for low, medium and high frequencies for driving a relatively low impedance transmission line having an input terminal for conveying said video signal to utilization apparatus, comprising in combination:

means coupling said input transistor collector electrode to said output transistor base electrode;

a second load resistor connected to said output transistor emitter electrode;

an output coupling capacitor having an input terminal connected to said output transistor emitter electrode and an output terminal coupled to the input terminal of said transmission line;

a feedback transistor having base, emitter and collector electrodes;

a DC feedback path including a resistor connected between said output transistor emitter electrode and said feedback transistor base electrode;

an AC feedback path including a capacitor connected from the output terminal of said coupling capacitor to said feedback transistor base electrode;

a third load resistor connected to said feedback transistor emitter electrode; and

a composite feedback resistor connected between said .feedback transistor emitter electrode and said input transistor base electrode.

References Cited UNITED STATES PATENTS ROY LAKE, Primary Examiner.

J. B. MULLINS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,356,959 December 5, 1967 Benjamin S. Vilkomerson It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 18, for "output" read input Signed and sealed this 18th day of March 1969.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A VIDEO SIGNAL AMPLIFIER HAVING SUBSTANTIALL EQUAL RESPONSE FOR ALL VIDEO SIGNAL FREQUENCIES FOR DRIVING A TRANSMISSION LINE HAVING AN INPUT TERMINAL FOR CONVEYING SAID VIDEO SIGNAL TO UTILIZATION APPARATUS, COMPRISING IN COMBINATION: A PAIR OF TRANSISTORS HAVING INPUT AND OUTPUT ELECTRODES AND A COMMON INPUT-OUTPUT ELECTRODE; SAID TRANSISTORS BEING INTERCONNECTED SO THAT SIGNALS DERIVED FROM SAID OUTPUT ELECTRODE ARE OF OPPOSITE PHASE TO SIGNALS APPLIED TO SAID INPUT ELECTRODE; MEANS FOR IMPRESSING UPON THE INPUT ELECTRODE OF SAID TRANSISTOR PAIR A VIDEO SIDNAL HAVING A RELATIVELY WIDE RANGE OF FREQUENCY COMPONENTS; AN OUTPUT COUPLING CAPACITOR HAVING AN INPUT TERMINAL CONNECTED TO SAID TRANSISTOR PAIR OUTPUT ELECTRODE AND AN OUTPUT TERMINAL COUPLED TO SAID TRANSMISSION LINE; A NEGATIVE FEEDBACK CIRCUIT BETWEEN THE OUTPUT AND INPUT ELECTRODES OF SAID TRANSISTOR PAIR; SAID CIRCUIT INCLUDING A FEEDBACK TRANSISTOR HAVING INPUT AND OUTPUT ELECTRODES; DC AND AC MEANS RESPECTIVELY COUPLING THE OUTPUT ELECTRODE OF SAID TRANSISTOR PAIR AND THE OUTPUT TERMINAL OF SAID COUPLING CAPACITOR TO SAID FEEDBACK TRANSISTOR INPUT ELECTRODE; AND MEANS COUPLING SAID FEEDBACK TRANSISTOR OUTPUT ELECTRODE TO SAID TRANSISTOR PAIR INPUT ELECTRODE. 